Residuum rare earth magnet

ABSTRACT

A permanent magnet for an MRI scanner is made by removing extraneous elements from an ore containing rare earth elements to leave elements Pr and Nd therein, and then selectively stripping therefrom a portion of the element Nd as a byproduct to leave an ore residuum including both elements Pr and Nd therein. The residuum is alloyed with a transition metal to form an alloy therewith. The alloy is then formed into a rare earth permanent magnet configured for use in the MRI scanner.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to MRI scanners, and,more specifically, to magnetic field generators therein.

[0002] A magnetic resonance imaging (MRI) system or scanner is commonlyused for precisely determining structure of organic molecules. A targetis placed in an imaging volume or zone under a strong magnetic field andanalyzed by the absorption and re-emission of radiofrequency-electromagnetic radiation by hydrogen or carbon nuclei. Theresonant frequency of this absorption and re-emission is a function ofthe gyromagnetic ratio of the nuclei and the applied magnetic field.

[0003] MRI imaging is a derivative of nuclear magnetic resonance (NMR)spectroscopy used by organic chemists to determine organic moleculestructure. In NMR spectroscopy, variations in emission intensity as afunction of frequency are used to infer variations in the structure ofthe organic molecule being examined. These frequency variations are dueto variations in the local magnetic field caused by variations in theelectronic and molecular structure of the organic molecule.

[0004] In MRI imaging, variations in emission intensity as a function offrequency are used to generate an image of the target which is typicallya selected portion of a human patient. Frequency is used to encodespatial address information. Variations in local magnetic field arecreated by a pulsed gradient coil system to give a discrete and slightlydifferent field and corresponding frequency for each volume element inthe field of view.

[0005] The applied magnetic field for NMR spectroscopy is substantiallyhigh, and requires a superconducting magnet. The applied magnetic fieldfor MRI imaging is substantially lower and is typically provided by asuperconducting magnet, and more recently by permanent magnets with evenlower magnetic field strength.

[0006] The use of permanent magnets in the magnetic field generators ofan MRI scanner substantially reduces the complexity and cost thereof.And, due to advances in improving resolution and image quality of MRIscanners, performance of permanent magnet-based MRI scanners has beenimproved.

[0007] Nevertheless, the relatively high magnetic field strengthrequired for MRI imaging requires a high performance permanent magnetsuch as rare earth permanent magnets having magnetic energy densitiessubstantially greater than conventional ferrite magnets for example. Thetypical high performance permanent magnet for MRI scanners is thesintered rare earth neodymium (Nd), iron (Fe), and boron (B) magnet.

[0008] The significant magnetic properties of the permanent magnet foran MRI application include the residual magnetic flux density (B_(r)),coercive force (H_(c)), intrinsic coercive force (H_(ci)), and maximumenergy product (BH)_(max).

[0009] The sintered NdFeB rare earth permanent magnet provides highperformance for use in various applications such as the MRI magneticfield generator, as well as for use in various portions of a computerincluding its hard drive and actuation motors. The composition of thepermanent magnet and the sequential processes from mine to finishedproduct are currently optimized for NdFeB to obtain the highest energyproduct (BH)_(max) and the highest intrinsic coercive force H_(ci).

[0010] However, the resulting high performance permanent magnet as usedfor MRI scanners requires well over a thousand kilograms thereof perscanner which is orders of magnitude greater than the small gram amountsthereof required for a typical computer. Accordingly, the cost of usingpermanent magnets in an MRI scanner is substantially high whichcorrespondingly limits the practical availability thereof.

[0011] The production of permanent magnets for the MRI scannernecessarily begins by initially mining the ore which contains a mixtureof various rare earth elements and other miscellaneous elements. Theparticular rare earth element of interest, such as Nd, must be refinedfrom the basic ore into a substantially pure form greater than about99%. The rare earth element is then alloyed with separately refinedelements such as iron and boron to form an alloy thereof. The alloy inpowder form is compacted under pressure in a magnetic field, and heatsintered to form blocks of permanent magnets which are magnetized andassembled in the required configuration for the magnetic field generatorof the MRI scanner. The remainder of the scanner is then assembled forcooperating with the permanent magnets.

[0012] The resulting cost of the MRI scanner includes in significantpart the corresponding high cost to process the rare earth ore forisolating the specific rare earth element followed in turn by alloyingthe rare earth element with iron and boron to produce the resulting rareearth permanent magnets.

[0013] Accordingly, it is desired to reduce the cost of a MRI scanner byreducing the cost of the rare earth permanent magnets therein, and thecosts in processing the rare earth elements thereof.

BRIEF SUMMARY OF THE INVENTION

[0014] A permanent magnet for an MRI scanner is made by removingextraneous elements from an ore containing rare earth elements to leaveelements Pr and Nd therein, and then selectively stripping therefrom aportion of the element Nd as a byproduct to leave an ore residuumincluding both elements Pr and Nd therein. The residuum is alloyed witha transition metal to form an alloy therewith. The alloy is then formedinto a rare earth permanent magnet configured for use in the MRIscanner.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic, elevational sectional view through an MRIscanner having rare earth permanent magnets therein in accordance withan exemplary embodiment of the present invention.

[0016]FIG. 2 is a top sectional view through the scanner illustrated inFIG. 1 and taken along jogged line 2-2.

[0017]FIG. 3 is a flowchart representation of a method for making theMRI scanner illustrated in FIGS. 1 and 2, including the permanentmagnets therein, in accordance with an exemplary embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0018] Illustrated schematically in FIG. 1 is an MRI imaging system orscanner 10 in accordance with an exemplary embodiment of the presentinvention. The scanner includes a magnetic field generator 12 includinga magnetic yoke 14, an opposing pair of magnetic field generating pads16 mounted to the yoke and spaced apart from each other, and a pair ofcooperating pole pieces 18 disposed adjacent to respective ones of thepads for shaping the magnetic field therefrom in a central imagingvolume or zone 20 therebetween.

[0019] The magnetic yoke 14 is conventional in configuration andincludes iron top and bottom plates against which respective ones of thepads 16 are disposed. The yoke also includes iron side posts joiningtogether the top and bottom plates for providing a magnetic circuitpath.

[0020] A target 22, such as a human patient, may be positioned withinimaging zone 20 for undergoing magnetic resonance imaging of selectedregions thereof. The magnetic field generating pads 16 are rare earthpermanent magnets in accordance with the present invention, and areconfigured for producing a substantially uniform magnetic field betweenthe opposing pads 16 and through the imaging zone 20. The uniformity ofthe magnetic field in the imaging zone 20 is shaped in part by the ironpole pieces 18 in a conventional manner.

[0021] A plurality of gradient coils 24 are disposed adjacent to thecorresponding pole pieces 18 for locally varying magnetic field in theimaging zone 20. The coils 24 are joined to corresponding gradient powersupplies 26. The gradient coils and power supplies therefor may take anyconventional form for effecting local magnetic gradient fields in threeorthogonal axes XYZ within the imaging zone. The gradient coils areexcited by pulses of electrical current from their power supplies tosuperimpose a slightly different incremental magnetic field in eachvolume element or voxel being examined in the imaging zone 20 to providea unique and known field and corresponding frequency address for eachvoxel.

[0022] A radio frequency (RF) coil 28 is disposed around the imagingzone 20 for radiating RF excitation energy therein for exciting hydrogennuclei in the target 22. A corresponding RF power supply 30 is joined tothe RF coil 28 for providing power thereof. An RF receiver 32 isoperatively joined with the RF coil 28 for receiving RF signals as thehydrogen nuclei release energy during MRI operation.

[0023] A suitable digitally programmable computer 34 is operativelyjoined to the power supplies 26,30 and receiver 32, and provides meansfor controlling the MRI system to magnetically resonate the target 22,interpret the signals received from the excited target 22, and createMRI scanning images therefrom in a conventional manner.

[0024] But for the rare earth permanent magnet pads 16, the entire MRIscanner 10 may be conventional in configuration and operation forscanning the target 22 in the imaging zone 20. The permanent magnet pads16 may be made in a new manner, with a correspondingly new composition,for substantially reducing the cost of manufacture of the MRI scannerwhile maintaining comparable imaging performance including image qualityand resolution.

[0025] Since the amount of permanent magnet material required in thepads 16 for an individual MRI scanner typically requires thousands ofkilograms of material, substantial cost reduction in the scanner may beobtained by correspondingly reducing the cost of making the permanentmagnets used therein. Furthermore, the resulting lower cost permanentmagnet pads 16 allow for increased volume thereof and furtherimprovements in the configuration thereof as shown in FIGS. 1 and 2 inan exemplary embodiment.

[0026] The cost of the MRI scanner may be reduced while maintainingcomparable performance thereof, or some of the cost reduction may beoffset for further increasing performance of the scanner by increasinguniformity of the applied magnetic field from the permanent magnets.

[0027] The permanent magnet pads 16 require a specific configuration andspecific composition for being effective in generating a uniformmagnetic field across the imaging zone 20 for use in magnetic resonanceimaging. Production of the pads, however, includes a substantial numberof steps from mining the rare earth containing ore, refining the ore,alloying the resulting metal with a suitable transition metal, andforming rare earth permanent magnets fabricated into the resulting pads16.

[0028] In accordance with the present invention, it has been discoveredthat the cost of each of the several process steps required foreventually producing the permanent magnet pads 16 tends to bemultiplicative with the cost of the preceding steps. If a preceding stepis costly, the succeeding steps tend to be correspondingly costly.Accordingly, by reducing the cost of reducing a processing step, costsof succeeding steps may be correspondingly reduced thusly accumulatingcost reductions over the entire process for substantial savings in finalcost of the MRI scanner.

[0029] For example, in the conventional production of rare earthpermanent magnets, the individual elements thereof are separatelyrefined to substantially pure form and then precisely alloyed togetherfor controlling the metallurgical composition thereof, metallurgicalmicrostructure thereof, and the resulting magnetic performance thereof.The typical high performance, rare earth permanent magnet found incurrent MRI scanners uses essentially pure neodymium alloyed with thetransition metal iron, and with boron to produce a NdFeB sintered rareearth permanent magnet. Additional, substantially pure elements may alsobe alloyed into the permanent magnet for improving magnetic propertiesthereof in a conventional manner.

[0030] Although there are several rare earth elements which may beindividually used for forming rare earth permanent magnets, neodymiumhas conventionally offered the highest magnetic performance for use inMRI scanners, as well as for other high performance applications such asthose typically associated with computers including the small drivemotors used therein. Since the weight of high performance permanentmagnets used in a computer application is on the order of grams, theassociated high cost therefor is a small contribution to the overallcost of the computer system. However, since an MRI scanner requiresthousands of kilograms of high performance permanent magnet material, acorrespondingly high cost thereof is a major cost contributor to theoverall cost of the scanner.

[0031] In accordance with the present invention, an improved process ofmaking the MRI permanent magnets is disclosed for substantially reducingthe cost thereof, and the corresponding costs of the MRI scanner itself,while obtaining comparable performance. The improved process results ina different composition of the rare earth permanent magnet, and permitsa change in configuration thereof for further improving magnetic fielduniformity in the scanner.

[0032]FIG. 3 illustrates in flowchart form a method or process formaking a rare earth permanent magnet 16 configured for use in the MRIscanner 10 of FIG. 1 in accordance with an exemplary embodiment of thepresent invention. The process starts at the mine from which a suitableore 36 is provided. The ore typically includes a combination of severalrare earth elements including cerium (Ce), lanthanum (La), Nd, andpraseodymium (Pr), with miscellaneous secondary elements. In oneexemplary composition, the rare earth ore includes 49% Ce, 33% La, 13%Nd, 4% Pr, and the remainder of miscellaneous elements.

[0033] The basic steps in processing rare earth containing ore areconventional, and culminate in the production of high purity Nd oxideand high purity Pr oxide separately removed from the ore with puritygreater than about 99.9%. This refining process includes many steps andcorrespondingly high cost for first removing extraneous elements andfinally separating the high purity Nd and Pr therefrom.

[0034] In accordance with a preferred embodiment, extraneous elementsare firstly removed from the ore 36 to leave the elements Ce, Pr, and Ndtherein. This may be accomplished using conventional process steps.

[0035] For example, the initial ore is processed to separate theextraneous elements therein not required for liberating the desired rareearth elements. The ore may be processed using roasting, leaching,flotation, and solvent extraction, for example, for removing undesirediron, lead (Pb), thorium (Th), samarium (Sm), gadolinium (Gd), andeuropium (Eu).

[0036] In particular, the element Ce is preferably only partiallyremoved from the ore, with the remaining rare earth (Re) elements in theintermediate ore 36 a being converted to chloride with a resultingcomposition of LaCePrNdSm from which are removed oxides of Sm, Gd, Eu.From the resulting mixture of LaCePrNd, the oxide of LaCe is removedleaving an intermediate ore 36 b in solution.

[0037] Although these process steps are basically conventional, asignificant departure therefrom in accordance with a preferredembodiment is the partial removal of cerium, in oxide form, for reducingthe cerium component of the rare earth elements in the intermediatemixture 36 a to an amount greater than about 0.6%. In conventionalpractice, substantially all the cerium is removed to an amount less than0.6% of the rare earth elements so that the resulting refined rare earthelements are substantially pure.

[0038] It has been discovered that the separation of cerium from therare earth elements is a prime contributor to the cost of the rare earthrefining. However, the introduction of cerium in the resulting permanentmagnet correspondingly reduces the intrinsic coercive force H_(ci)significantly. The rare earth magnet without cerium can achieve amaximum intrinsic coercive force H_(ci) of up to about 15 kOe.

[0039] For satisfactory performance of the permanent magnet for the MRIscanner, removal of cerium from the rare earth elements may be limitedfor maintaining the cerium component of the rare earth elements up toabout 10%. At 10% cerium content, the resulting permanent magnet willhave an intrinsic coercive force H_(ci) of about 7 kOe.

[0040] In a preferred embodiment, however, the element Ce is partiallyremoved from the ore to reduce the cerium component of the rare earthelements to about 5% for achieving an intrinsic coercive force H_(ci) ofabout 9 kOe, or greater, for obtaining suitable performance of thepermanent magnet for the MRI application.

[0041] Accordingly, the various extraneous elements are removed from theore to leave primarily only the elements Ce, Pr, and Nd therein in theintermediate ore 36 b. The discovery of retaining this significantcomponent of the element Ce in the intermediate ore, allows asubstantial reduction in cost of refining the rare earth elements, whileobtaining acceptable magnetic performance.

[0042] In accordance with another feature of the present invention,instead of individually removing the rare earth elements Pr and Nd fromthe intermediate ore to produce substantially pure forms thereof as isconventionally done, only a portion of the rare earth element Nd isselectively stripped from the intermediate ore as a byproduct 38, inoxide form, to leave an ore residuum 36 c, in mixed oxide form,including the elements Ce, Pr, and Nd. In the preferred embodiment, theprocessed ore residuum 36 c consists essentially only of the elementsCe, Pr, and Nd.

[0043] Of particular significance is that the rare earth element Pr isnot individually stripped from the intermediate ore and remains in theresiduum 36 c. The residuum therefore includes both the rare earthelements Pr and Nd, with only a portion of the element Nd beingselectively stripped to form the essentially pure Nd byproduct 38.Accordingly, the residuum 36 c includes a fraction F of the element Nd,and the byproduct 38 includes the complement, i.e., 1 −F, of the elementNd fraction.

[0044] Selective stripping may be accomplished in various conventionalprocesses. For example, the intermediate ore 36 b containing CePrNd maybe processed in an organic solution from which these elements arecollectively stripped by solvent extraction, and precipitated as mixedoxalate or carbonate salts. Stripping of the element Nd may be effectedusing additional separation or extraction stages to isolate the Ndbyproduct.

[0045] The resulting CePrNd mixed oxide residuum 36 c is thereforedepleted of only a portion of its Nd component, which correspondinglyincreases the relatively percentage of the rare earth element Prtherein.

[0046] The mixed oxide residuum is then converted from oxide to metal inany conventional manner, with the mixed rare earth metal residuum beingalloyed with a transition metal, such as iron, to form a metal alloy 36d therewith. The residuum alloy 36 d is then suitably formed into amixed rare earth permanent magnet, such as in the form of the permanentmagnet pads 16 for MRI scanner.

[0047] In the preferred embodiment, the residuum 36 c is alloyed withboth iron and boron to form a mixed rare earth permanent magnetcomprising CePrNdFeB. Additional, substantially pure elements may alsobe alloyed into the permanent magnet for improving magnetic propertiesthereof in a conventional manner.

[0048] Correspondingly, the substantially pure Nd byproduct 38 may beused for various other purposes such as in computer applicationsrequiring high performance rare earth permanent magnets. The Ndbyproduct is converted from oxide to metal and alloyed in anyconventional manner with a transition metal, such as iron, and withboron to form an alloy therewith comprising NdFeB. The byproduct alloy40 is then suitably formed into a unitary rare earth (Nd) permanentmagnet 40 a for use in a byproduct application requiring highperformance rare earth permanent magnets. Additional, substantially pureelements may also be alloyed into the permanent magnet for improvingmagnetic properties thereof in a conventional manner.

[0049] The rare earth permanent magnets of either unitary or mixed rareearth composition may be formed in any conventional manner. For example,the processed rare earth oxides are converted to metal which aretypically melted in a vacuum furnace. The resulting metals are crushed,pulverized, and milled in an inert atmosphere such as nitrogen, and jetmilled with nitrogen to a micron sized powder. The components of thepermanent magnets are suitably mixed and blended to final composition,and subjected to magnetic field alignment for undergoing die orisostatic pressing. The pressed magnet material is then sintered andheat treated in a suitable vacuum or inert gas furnace. The resultingpermanent magnetic material is cut or machined to desired size andconfiguration such as in block form. The blocks are then magnetized andassembled into the required configuration such as the pads 16 for use inthe scanner.

[0050] Whereas the Nd byproduct 38 may be alloyed with iron and boron toform a high performance NdFeB sintered permanent magnet, the mixed oxideresiduum 36 c may be alloyed with iron and boron to form a sinteredmixed rare earth CePrNdFeB permanent magnet having different compositionand magnetic properties. As indicated above, intrinsic coercive forceH_(ci) is a significant magnetic property which decreases withincreasing cerium content of the permanent magnet. Furthermore, althoughthe rare earth elements Nd Pr are different, the combination thereof inthe mixed rare earth permanent magnet does not adversely affect theintrinsic coercive force.

[0051] Evaluation of the interrelated effects of the four significantmagnetic properties including residual magnetic flux density, coerciveforce, intrinsic coercive force, and maximum energy product indicatesthat the nominal amount of cerium as described above, and retentiontogether of the rare earth elements Nd and Pr provide acceptablemagnetic performance of the mixed rare earth permanent magnet for theMRI scanner with a significant maximum energy product (BH)_(max) withina range of about 36-40 MGOe.

[0052] In the preferred embodiment, the element Nd fraction F in theresiduum 36 c is less than the element Nd complement (1−F) in thebyproduct 38. Preferably, the element Nd fraction F in the residuum andresulting permanent magnet is up to about 0.11 (11%). This fraction isbased on a cost analysis wherein the relative cost of the mixed rareearth oxide processing increases nonlinearly with an increase in theelement Nd fraction F, with the fraction F equaling 0.11 providing asubstantial reduction in cost of the mixed rare earth permanent magnetwhile achieving a mixed rare earth composition having suitable magneticproperties for use in the MRI scanner.

[0053] As indicated above, the substantial reduction in processing costattributed to not removing all of the cerium from the rare earth ore,and by selectively stripping only a portion of the element Nd from theore to leave the mixed rare earth residuum provides additional costreductions in subsequent steps of the manufacturing process leading tothe final assembly of the MRI scanner.

[0054] The scanner is accordingly manufactured by initially forming thepermanent magnet pad 16 in the magnetic field generator 12 from the ore36 containing rare earth elements including Pr and Nd by selectivelystripping therefrom the element Nd as the byproduct 38 to leave theresiduum 36 c including both elements Pr and Nd therein. The residuum issubsequently alloyed with the transition metal, such as iron, and withboron to form the mixed rare earth permanent magnet.

[0055] The permanent magnets are preferably formed in unitary magnetblocks which may be suitably assembled into the pair of magnetic fieldgenerator pads 16 on opposite sides of the magnetic yoke 14 illustratedin FIG. 1. The pair of pole pieces 18 are then assembled adjacent to thecorresponding pads 16 for shaping the magnetic field therefrom in theimaging zone 20 therebetween.

[0056] The gradient coils 24 are assembled adjacent to the respectivepole pieces 18 for locally varying the magnetic field in the imagingzone 20. The RF coil 28 is assembled around the imaging zone 20 forradiating excitation energy therein. And, the gradient coils 24 and RFcoil 28 are operatively joined to the computer 34 and theircorresponding power supplies for magnetically resonating the target 22in the imaging zone 20 for imaging thereof in a conventional manner.

[0057] In the preferred embodiment, the element Ce is partially removedfrom the rare earth ore prior to the selective stripping process toreduce the Ce component of the rare earth elements to greater than 0.6%and up to about 10%, with about 5% being preferred.

[0058] As indicated above, the extraneous elements are removed from theore 36 prior to selective stripping for leaving primarily only Ce Pr Ndtherein from which the single rare earth byproduct 38 and the mixed rareearth residuum are stripped.

[0059] In view of the substantial cost reduction associated with theproduction of the mixed rare earth permanent magnet pads 16, includingCePrNdFeB, the configuration of the pads 16 may be economically changedfor enhancing performance of the MRI scanner 10. For example, the sizeand mass of the permanent magnet pads 16 may be increased compared toconventionally sized NdFeB permanent magnet pads for improvingperformance without a substantial increase in corresponding cost.

[0060] As shown in the exemplary embodiment of FIGS. 1 and 2, the pads16 and pole pieces 18 are annular and coaxially aligned with each other,with the imaging zone 20 being defined centrally therebetween. Inconventional practice, the permanent magnet pads would have asubstantially constant thickness and cooperate with the specificallyconfigured pole pieces 18 for maximizing the uniformity of the magneticfield extending between the opposing pole pieces and pads. Nevertheless,the applied magnetic field across the imaging zone 20 varies slightlyfrom point to point with about 10-20 parts per million.

[0061] In order to further increase the uniformity of the appliedmagnetic field from the permanent magnet pads 16 with an even smallervariation in parts per million, the pad 16 as illustrated in FIG. 1 ispreferably selectively non-uniform in thickness A. Preferably, the pads16 are thicker at their perimeters than at their middle section forincreasing uniformity of the magnetic field in the imaging zone 20. Inparticular, the axial magnetic field between the opposing pads 16 enjoysincreased uniformity along the radial direction.

[0062] The actual configuration of the permanent magnet pads 16 may beobtained by conventional multidimensional computer analytical techniquesfor maximizing the magnetic field uniformity in the imaging zone 20. Thecost constraint on fabricating the pads 16 is ameliorated in accordancewith the new manufacturing process, thusly allowing an increased amountof permanent magnet material in the pads selectively positioned forfurther improving MRI imaging.

[0063] Correspondingly, the pole pieces 18 may be optimized inconfiguration for maximizing the uniformity of the applied magneticfield from the corresponding pads 16 within the imaging zone 20. And,additional freedom is obtained in designing the pole pieces 18 tocooperate both with the pads 16 and the gradient coils 24.

[0064] The selective strip process described above for forming sintered,mixed rare earth permanent magnets produces high performance magnets forthe MRI scanner as well as substantially pure Nd byproduct for use inother high performance magnet applications requiring considerably lessmaterial weight. Retention of the rare earth cerium component in theresulting permanent magnet substantially reduces cost of themanufacturing process without adversely compromising magneticperformance. Selectively stripping the element Nd to leave the mixedrare earth residuum results in a mixed rare earth permanent magneteconomically obtained without the need for alloying substantially purerare earth elements separately refined in expensive processes.

[0065] While there have been described herein what are considered to bepreferred and exemplary embodiments of the present invention, othermodifications of the invention shall be apparent to those skilled in theart from the teachings herein, and it is, therefore, desired to besecured in the appended claims all such modifications as fall within thetrue spirit and scope of the invention.

[0066] Accordingly, what is desired to be secured by Letters Patent ofthe United States is the invention as defined and differentiated in thefollowing claims in which we claim:

1. A method of making a permanent magnet comprising: providing orecontaining rare earth elements including Ce, Nd, and Pr; partiallyremoving said element Ce from said ore; removing extraneous elementsfrom said ore to leave elements Ce, Pr, and Nd therein; selectivelystripping from said ore a portion of said element Nd as a byproduct toleave an ore residuum including elements Ce, Pr, and Nd; alloying saidresiduum with a transition metal to form an alloy therewith; and formingsaid residuum alloy into a mixed rare earth permanent magnet.
 2. Amethod according to claim 1 wherein said element Pr is not stripped fromsaid ore and remains in said residuum.
 3. A method according to claim 2wherein said residuum includes a fraction of said element Nd, and saidbyproduct includes the complement of said element Nd fraction.
 4. Amethod according to claim 2 further comprising: alloying said byproductwith a transition metal to form an alloy therewith; and forming saidbyproduct alloy into a unitary rare earth permanent magnet.
 5. A methodaccording to claim 4 wherein said transition metal is iron, and bothsaid residuum and byproduct are alloyed with both iron and boron torespectively form CePrNdFeB and NdFeB rare earth permanent magnets.
 6. Amethod according to claim 3 wherein said element Ce is partially removedfrom said ore to reduce said Ce component of said rare earth elements togreater than about 0.6%.
 7. A method according to claim 6 wherein saidelement Ce is partially removed from said ore to reduce said Cecomponent of said rare earth elements to less than about 10%.
 8. Amethod according to claim 6 wherein said element Ce is partially removedfrom said ore to reduce said Ce component of said rare earth elements toabout 5%.
 9. A method according to claim 3 wherein said element Ndfraction in said residuum is less than said element Nd complement insaid byproduct.
 10. A method according to claim 9 wherein said elementNd fraction in said residuum is up to about 0.11 in value.
 11. A methodaccording to claim 3 further comprising forming said permanent magnetinto a pair of magnetic field generator pads configured for a magneticresonance imaging scanner.
 12. A method according to claim 11 furthercomprising: mounting said permanent magnet pads to a magnetic yokespaced apart from each other; and mounting respective pole piecesadjacent to said pads for shaping magnetic field in an imaging zonetherebetween.
 13. A method according to claim 12 wherein said pads andpole pieces are annular and coaxially aligned, and said pads are thickerat perimeters thereof for increasing uniformity of said magnetic fieldin said imaging zone.
 14. A method according to claim 1 3 furthercomprising: mounting a plurality of gradient coils adjacent to said polepieces for locally varying said magnetic field in said imaging zone;mounting an RF coil 28 around said imaging zone for radiating excitationenergy therein; and operatively joining said gradient coils and RF coilto a computer for magnetically resonating a target in said imaging zonefor imaging thereof.
 15. A method of making a permanent magnetcomprising: removing extraneous elements from ore containing rare earthelements to leave elements Pr and Nd therein; selectively stripping fromsaid ore a portion of said element Nd as a byproduct 38 to leave an oreresiduum including both elements Pr and Nd therein; alloying saidresiduum with a transition metal to form an alloy therewith; and formingsaid residuum alloy into a mixed rare earth permanent magnet.
 16. Amethod according to claim 15 further comprising: alloying said byproductwith a transition metal to form an alloy therewith; and forming saidbyproduct alloy into a unitary rare earth permanent magnet.
 17. A methodaccording to claim 16 wherein said transition metal is iron, and bothsaid residuum and byproduct are alloyed with both iron and boron torespectively form CePrNdFeB and NdFeB rare earth permanent magnets. 18.A method according to claim 16 wherein said element Ce is partiallyremoved from said ore to reduce said Ce component of said rare earthelements to greater than about 0.6%.
 19. A method of making a magneticimaging resonance scanner including a permanent magnet field generatorcomprising: forming permanent magnet in said generator from orecontaining rare earth elements including Pr and Nd by selectivelystripping therefrom said element Nd as a byproduct to leave a residuumincluding both elements Pr and Nd therein, and alloying said residuumwith a transition metal and boron to form a mixed rare earth permanentmagnet; assembling said permanent magnets as a pair of spaced apart padson opposite sides of a magnetic yoke; assembling a pair of pole piecesadjacent said pads for shaping magnetic field therefrom in an imagingzone therebetween; assembling a plurality of gradient coils adjacentsaid pole pieces for locally varying magnetic field in said imagingzone; assembling an RF coil adjacent said imaging zone for radiatingexcitation energy therein; and operatively joining said gradient coilsand RF coil to a computer for magnetically resonating a target in saidimaging zone for imaging thereof.
 20. A method according to claim 19further comprising partially removing cerium from said ore to reducesaid cerium component of said rare earth elements to greater than about0.6%.
 21. A method according to claim 20 further comprising removingextraneous elements from said ore to leave cerium, and elements Pr andNd therein prior to said selective stripping.
 22. A magnetic resonanceimaging scanner comprising: a magnetic yoke; a pair of magnetic fieldgenerating pads mounted to said yoke and spaced apart from each other,said pads being rare earth permanent magnets including elements CePrNdalloyed with a transition metal and boron; a pair of pole piecesdisposed adjacent to respective ones of said pads for shaping magneticfield in an imaging zone therebetween; a plurality of gradient coilsdisposed adjacent to said pole pieces for locally varying magnetic fieldin said imaging zone; an RF coil disposed around said imaging zone forradiating excitation energy therein; and means operatively joined tosaid gradient coils and said RF coil for magnetically resonating atarget in said imaging zone for imaging thereof.
 23. A scanner accordingto claim 22 wherein said transition metal comprises iron, and saidpermanent magnets comprise CePrNdFeB.
 24. A scanner according to claim22 wherein said pads and pole pieces are annular and coaxially aligned,and said pads are thicker at perimeters thereof for increasinguniformity of said magnetic field in said imaging zone.